1. Field of the Invention
The present invention relates to high selectivity aqueous slurries for the Chemical Mechanical Polishing/Planarization of substrates. The slurries of the present invention are useful for polishing an insulator film of a semiconductor device and in particular for processing shallow trench isolation structures on semiconductor substrates. The slurries include an aqueous medium, non-modified colloidal silica abrasive particles and one or more organic compounds which provide removal of silicon oxide selectively over silicon nitride.
2. Description of Related Art
Chemical Mechanical Polishing/Planarization (CMP) has become a useful technology in the field of integrated circuits (IC) fabrication, where the surface of the semiconductor substrate is planarized and subsequent circuit patterns are laid down. Although polishing and planarization may be utilized interchangeably herein, it will be recognized by those skilled in the art that CMP encompasses both. Global planarization of topological features is commonly utilized in the manufacturing of high performance ultra-large scale (ULSI) devices. Structures having increasingly smaller device dimensions, increasing packaging density and multiple metal-insulator wiring levels impose stringent demands on planarity. Depth-of-focus requirements of photolithography masking processes put additional demands on topographical height variation, as non-planarity impacts on both IC device yield and performance.
Shallow Trench Isolation (STI) has become an enabling technology in the IC fabrication, where the number of neighboring transistors have been increased and the planarity has been improved, thereby replacing the Local Oxidation of Silicon (LOCOS) technique for sub-250 nm devices.
In STI processes, isolation trenches are plasma etched in the silicon utilizing a silicon nitride etch mask, and overfilling the trenches with silicon oxide, deposited via high density plasma chemical vapor deposition (HDP CVD) or plasma enhanced CVD (PECVD). Thereafter, the oxide is polished back to a planar surface via CMP. The sacrificial silicon nitride layer is chemically stripped and active devices are fabricated in the exposed silicon regions.
The STI CMP process performance is characterized by the post-polish trench oxide thickness and active area silicon nitride thickness, as well as within-die (WID) and within-wafer (WIW) thickness ranges. This set of parameters ultimately determines the height between the active silicon area of the transistor and the oxide in the trench, after the nitride stop layer has been removed.
Ideally the CMP process stops on the nitride barrier layer. However, due to the local polishing differences, small features polish faster than large features, whereas removal rate is inversely proportional to the pattern density (i.e., oxide in narrow trenches, polish slower than in wider trenches). This results in so-called nitride erosion (e.g., WID nitride thickness variation between active area feature of varying size and pattern density). Field oxide loss or so-called trench oxide dishing is another CMP related problem having a negative impact on the surface planarity.
Different approaches and integration solutions have been developed to decrease the pattern dependency impact of STI CMP, such as adding dummy features, varying pad stack, using fixed-abrasive pads, as well as slurry engineering.
One of the effective integration approaches to the direct STI CMP is based on using high selectivity slurry. The selectivity of a STI slurry is defined as the ratio of the material removal rate (RR) of oxide to that of silicon nitride.
One type of high-selectivity slurry system proposed for the STI CMP process has been the ceria-based slurry. Ceria particles are utilized as an abrasive component in an aqueous system due to their high rate of removal at low particle concentration. For example, U.S. Pat. Nos. 5,738,800, 6,042,714, 6,132,637 and 6,218,305 to Hosali et al disclose a polishing slurry composition for polishing a composite of silicon dioxide and silicon nitride. The slurry includes up to 5 weight percent ceria, a compound which complexes with silicon dioxide and silicon nitride and a surfactant affecting the silicon nitride removal.
Another ceria-based STI slurry composition is disclosed in U.S. Patent Application Publication Nos. 20020195421 and 20030006397 to Srinivasan et al. The combination of ceria abrasives with amino acids as selectivity enhancing compounds is claimed to provide oxide/nitride selectivity of 100 and higher for blanket films.
U.S. Pat. Nos. 6,114,249 and 6,436,835 describe a composition containing cerium oxide and a water-soluble organic compound having at least one group of —COOH, —COOMx (wherein Mx is an atom or a functional group capable of substituting an H atom to form a slat), —SO3H or SO3My (wherein My is an atom or a functional group capable of substituting an H atom to form a salt).
Commercially available ceria-based slurries include the Hitachi HS-8005 slurry combined with HS-8102GP (a proprietary selectivity additive). One of the disadvantages associated with the latter slurry is that it is a two-component system, which requires mixing prior to employing it. The mixing ratio governs the slurry performance including the removal rate, selectivity and dishing. Furthermore, handling and pot life (i.e., slurry residence after mixing the components) has been found to impact process performance. Therefore, mixing the components becomes of utmost importance, requiring utilization of special mix-in-place systems, which provide precise mixing and minimize the slurry residence time. See, T-C. Tseng, et al., STI CMP Process with High-Selectivity Slurry, Proceedings of 2002 CMP-MIC, pp. 255–259; Benjamin A. Bonner, et al., Development of a Direct Polish Process for Shallow Trench Isolation Modules, Proceedings of 2001 CMP-MIC, pp. 572–579).
A further disadvantage associated with the aforementioned ceria based slurries is that ceria is an abrasive powder of CeO2. This powder has a significantly lower suspension stability, as compared to colloidal particles. Therefore, the ceria-based slurries require to be maintained in a state of agitation to prevent sedimentation and packing of the particles.
Further, utilizing powder-derived ceria abrasive particles which are harder than colloidal silica particles, have larger mean sizes and irregular morphology, results in decreased quality of the polished wafer surface including, but not limited to higher surface roughness and defects. Additionally, the formation of microscratches on the wafer surface is a great concern when using ceria-based slurries.
Another type of ceria-based system for STI CMP is the so-called “topography selective” slurries. See, Patrick Leduc, et al., CMP: Aiming for Perfect Planarization, Proceedings of 2002 CMP-MIC, pp. 239–246. In general, topography selective slurries belong to the class of so-called “pressure sensitive slurries”. These slurries are broadly defined as having non-linear (i.e., non-Prestonian) relationship between the rate of removal and pressure. These slurries are considered to have a different mechanism of interaction between the abrasive particles, additives and polished films. They exhibit very low oxide removal on blanket wafers, but the removal rate increases on patterned wafers having a pronounced topography (i.e., difference in local pressures).
In spite of very good planarization efficiency and low nitride erosion reported when using this STI CMP slurry, there is the major disadvantage associated with the topography selective slurries—the very strong dependency of oxide polishing rate on the mask layout. Careful control on the overfill oxide thickness is required for the wafer to be completely cleared. See, Ping-Ho Lo et al., Characterization of Selective-CMP, Dummy Pattern and Reverse Mask Approaches in STI Planarization Process, Proceedings of 1999 CMP-MIC, pp. 333–335.
There is difficulty removing residual oxide (e.g., when the topography becomes less pronounced) and the oxide removal rate significantly decreases for patterns with low density. Accordingly, these slurries are not efficient for particular STI formation processes, which do not generate explicit step height.
Several slurry systems based on colloidal silica particles have been proposed for STI CMP. Colloidal silica slurries are stable against settling, thereby eliminating need for agitating the system so as to prevent sedimentation, agglomeration and hard packing. Moreover, the smaller particle size and spherical morphology reduces the defects (i.e., microscratches) that may occur on the wafer surface when utilizing ceria abrasive particles.
Because of these advantages, some semiconductor chip manufacturers employ conventional, non-selective colloidal silica based slurries for STI-CMP. These slurries provide oxide/nitride selectivity ratios from 3 to 4 in blanket films; this selectivity might further be reduced on patterned wafers. Frequently this results in over-polish, under-polish and non-uniform thickness. To overcome these problems, slower polishing rates, stringent process and end-point controls are applied to achieve required product quality. These measures reduce efficiency, throughput and increase manufacturing cost. Significant efforts have been undertaken to increase selectivity of these slurries.
U.S. Pat. No. 4,525,631 to Silvestri et al describes a slurry containing about 6 weight percent colloidal silica adjusted to a pH of about 12 with KOH. The slurry provides oxide/nitride selectivity of about 10. Similarly, U.S. Pat. No. 6,019,806 to Sees et al discloses an approach to enhancing selectivity by increasing the pH of colloidal silica to about 13. The highest selectivity attained was about 15.
U.S. Pat. No. 5,671,851 to Beyer et al describes adding small amounts of Na salt of dichoroisocyanuric acid and sodium carbonate to attain a selectivity of 6.2. U.S. Pat. No. 6,114,249 to Canaperi et al discloses a colloidal silica slurry containing trietnolamine to attain a selectivity of 28.
U.S. Pat. No. 5,759,917 describes an STI CMP slurry including fumed silica particles, a carboxylic acid and a soluble cerium compounds. As shown in the examples, the selectivity is less than 20.
European Patent Publication 0 846 740 to Morrison et al disclose the selectivity of conventional colloidal silica slurry increased to as high as 30 by adding tetramethyl ammonium hydroxide and hydrogen peroxide.
Some of the disadvantages associated with the related colloidal silica slurries include the selectivity not being high enough, particularly when polishing patterned wafers. Integration issues for advanced generations of IC devices require very low oxide dishing and nitride loss. As device pitch continues to shrink, isolation trench aspect ratio increases and nitride layers become thinner. This reduces the margin for nitride loss during STI CMP process.
To overcome the disadvantages associated with the related art slurries and to meet the high planarity requirement for small feature size devices, a high selectivity colloidal silica based slurry is provided.
Another object of the invention is to provide a one-component, ready to use slurry system.
It is a further object of the invention to provide a slurry system for producing a high throughput for advanced devices, yet have an end-point control through selectivity.
It is yet another object of the invention to provide a non-modified colloidal silica based slurry, which minimizes the silicon nitride erosion.
Other objects and advantages of the invention will become apparent to one skilled in the art on a review of the specification, figures and claims appended hereto.